Step 1

Place a rack in middle of oven; preheat to 325°. Generously grease bottom and sides of springform pan with butter. Melt 4 Tbsp. butter in a small saucepan over medium heat until bubbling, about 2 minutes. Add brown sugar and ¼ tsp. Diamond Crystal or Morton kosher salt; cook, stirring constantly, until brown sugar is dissolved and butter is copper colored, about 30 seconds. (It’s okay if mixture doesn’t come together at this point.) Pour caramel into prepared pan.

Step 2

Carefully arrange bananas, cut side down, in pan, cutting them to fit in an even layer.Whisk together flour, almond meal, baking powder, and remaining ¾ tsp. Diamond Crystal or ½ tsp. Morton kosher salt in a medium bowl. Whisk egg, egg yolk, coconut milk, oil, vanilla, and 1 cup granulated sugar in a large bowl to combine. Add dry ingredients and mix with a rubber spatula until just combined. Pour batter over bananas and gently tap pan on counter to release any air bubbles.

Step 3

Place pan on a foil-lined rimmed baking sheet and transfer to oven. Bake cake, rotating pan halfway through, until golden brown and a tester inserted into the center comes out clean, 45–60 minutes. Immediately run an offset spatula around edges of pan to loosen cake. Transfer pan to a wire rack and let cake cool in pan 20 minutes; reserve baking sheet.

Step 4

Invert baking sheet over top of pan, then flip cake over onto baking sheet. Remove ring from pan; carefully lift away pan bottom, running offset spatula between bananas and bottom to release (don’t worry if some of the caramel sticks).

Step 5

Sprinkle remaining ¼ cup granulated sugar evenly over cake. Using kitchen torch, melt sugar until deep amber. Sprinkle cake with sea salt.